A hard disk drive system includes a rotating magnetic disk having information stored thereon, the information including servo burst information. A read head is movably supported adjacent the magnetic disk, for reading the information from the disk. A servo burst demodulation circuit is provided to process a servo burst signal generated by the read head in response to the servo information on the disk, in order to determine a strength or burst value of the servo burst signal from the read head.
One technique for demodulating the servo burst signal is to rectify the burst signal and to then apply it to an analog circuit which includes a capacitor. The capacitor is slowly charged during the duration of the rectified servo burst signal, the resulting level of charge on the capacitor being representative of the strength or burst value of the servo burst signal. In effect, this approach involves analog integration of the burst signal, in order to determine its total energy.
However, the servo burst signal may have abnormalities, for example due to problems such as dropins, drop-outs, fading, thermal asperity, and burst signal asymmetry. These abnormalities can produce erroneous results, by causing the capacitor to be undercharged or overcharged. In this regard, the level of charge on the capacitor is particularly sensitive to the last portion of a burst signal, where a noise spike or the like can cause the capacitor to be significantly overcharged in a very short time interval. Thus, this known approach of charging a capacitor involves sensitivity to noise and signal irregularities. Further, it requires a relatively significant amount of analog circuitry in the read channel.
Another known technique is to oversample the burst signal and to add up the absolute value of all the samples for the entire burst signal. In effect, this approach involves digital integration of the burst signal, in order to determine its total energy. However, this approach is also highly sensitive to signal irregularities of the type discussed above.
Yet another known approach is to carry out an analog median filtering, in particular by implementing an analog peak detection for each cycle of the servo burst signal, by switching the rectified analog burst signal successively to many different capacitors during respective cycles of the servo burst signal, and then selecting in the analog domain the capacitor having a charge level representing the median of the charge levels on all of the capacitors. The voltage on the selected capacitor is then digitized to convert the median value to the digital domain. This approach requires a substantial amount of circuitry, including the capacitors, the circuitry for switching the analog signal among the capacitors, and the circuitry for determining the median value of the charge levels on the capacitors. Further, it typically requires special analog circuitry for determining the zero-crossing points of the burst signal, in order to synchronize the switching among the capacitors to the burst signal. Moreover, the capacitors which are charged at the beginning of the servo burst signal will have a charge level that tends to decay during the remainder of the servo burst signal, thereby introducing inaccuracy into the subsequent attempt to determine a median value.
Although these known approaches have been generally adequate for their intended purpose, they have not been satisfactory in all respects. Some are sensitive to noise, and none are sufficiently effective in overcoming problems due to drop-ins, drop-outs, fading, signal asymmetries, and thermal asperity. The circuitry required can be relatively complex, and provides little or no flexibility for fine tuning the particular approach to a specific application.